You can read about combinators in the [Sitepoint reference or [URL="http://reference.sitepoint.com/css/combinators"]more specifically here](http://reference.sitepoint.com/css/selectorref).

If the question is about the child selector of which the combinator is the > sign then that simply matches all elements that are the immediate children of a specified element whereas a descendant selector matches all children.

The child selector is useful in dropdown menus because you limit the scope to the immediate children and not interfere with any nested submenus of which their could be many.